1. Field of the Invention
The present invention concerns the field of amplifying optical fibers and of optical fiber amplifiers.
2. Description of the Prior Art
There is represented diagrammatically in FIG. 1 an optical fiber amplifier. This amplifier 2 comprises an input port 4 and an output port 6 intended to be connected to a line fiber. The amplifier consists of a doped optical fiber 8, a pumping source 10 and an optical coupler 12. The pumping source is generally a laser and delivers a pumping optical signal that is injected into the doped fiber 8 by the optical coupler 12. This pumping optical signal is absorbed by the dopant, chosen from the rare earths and in particular erbium, present in the core of the doped optical fiber which, on becoming de-excited, transfers a portion of the absorbed energy to the optical signal propagating between the ports 4 and 6, thereby amplifying that optical signal.
The doped optical fiber may comprise a monomode core in which the dopant is found and into which the pumping optical signal is injected. It may also comprise a monomode core receiving the dopant and a multimode core surrounding the monomode core to receive the pumping signal. This second embodiment has the advantage of enabling the injection of a more powerful pumping signal and of producing better coupling between the pumping optical signal and the dopant.
Moreover, it is known that it is possible to choose the wavelength of the pumping signal by using quantum dots, and in particular by choosing the type of quantum dot and the size of the quantum dots, as indicated for example in the paper “Highly luminescent silicon nanocrystal with discrete optical transitions” J. D. Holmes et al., J. Am. Chem. Soc. 123 (2001) pp 3743-3748.
However, the quantum dots have a very large absorption section. Accordingly, in the case of a monomode optical fiber, the pumping signal is absorbed by the quantum dots over a length of fiber of the order of 50 μm. It is not possible with the current technology to insert a sufficient concentration of dopant over this short a distance, so that it is not possible to transfer the energy absorbed by the quantum dots to the dopant efficaciously.
To overcome this difficulty, there is proposed in the paper “Optical gain at 1.5 μm in nanocrystal Si sensitized, Er-doped silica waveguide using top-doping 470 nm LED”, J. Lee et al, OFC '04, PD19 a device as represented in FIG. 2. That device includes a flat waveguide 14 including quantum dots of Si and a dopant Er placed between two line fiber elements 16 and 18 and a strip 20 of photodiodes. The strip 20 delivers a pumping signal 22 transversely to the flat waveguide, and so the pumping signal is injected over a sufficient length of the flat waveguide. The length of the flat waveguide is 11 mm in the example described.
This solution is not yet satisfactory, however. In fact, it is clear that the waveguide cannot be very long (its length is of the same order of magnitude as the length of the strip of photodiodes), so that it remains necessary to dope the waveguide strongly. This causes cooperation between the ions of the dopant, which greatly reduces the efficacy of the optical conversion and therefore the amplification of the optical signal to be amplified.
Moreover, a coupling loss cannot be avoided between a flat waveguide and a circular line fiber.
An object of the invention is to alleviate the drawbacks of the prior art in the case of an amplifying optical fiber comprising quantum dots.